In many gas production environments, there are often heavier hydrocarbon compounds such as gas liquids, condensate, or even oils produced with the gas. In gas processing applications that remove contaminants from the gas stream, such as glycol dehydration, amine sweetening, or MEG reclamation, these heavier hydrocarbons often contaminate the process by becoming dissolved or suspended in the process fluids. In the oil and gas industry, triethylene glycol (TEG) is commonly used for gas drying, amine systems are commonly used for gas sweetening (hydrogen sulfide or carbon monoxide removal), and monoethylene glycol (MEG) is commonly used to prevent the formation of gas hydrates in pipelines. Hydrocarbon compounds require removal from each type of process because of the deleterious effects on the process. Although detailed descriptions are provided for MEG reclamation, the hydrocarbon removal process is similar for each gas processing application.
MEG is injected at the wellhead and readily mixes with the produced water to form a mixture referred to as rich MEG. The oil and rich MEG are separated in the final production separator. However, depending upon the properties of the oil, the hydrocarbon content of the rich MEG may be as high as 1,000 parts per million (ppm). When the oil properties are in the range of a light condensate, the hydrocarbon content may range from 100 ppm to 200 ppm, but even these concentrations may be excessive for optimal operation of the MEG reclamation process.
Standard gravity/time based process separation is the present technology for separating hydrocarbons from the rich MEG feed stream. The feed stream is commonly heated to 100° F. to 150° F. to facilitate this separation. Additional separation or particulate filtration may also be used to clean the rich MEG feed stream before it enters the reclamation process. Such filtration can be moderately successful depending on the design of the valves, piping, and vessel internals. However, it is still uncommon for these additional steps to reduce the hydrocarbon levels below 200 ppm.
Increasing the removal of hydrocarbons from the rich MEG improves the operation of the MEG reclamation process. In addition, hydrocarbons that are not removed from the rich MEG before the reclamation process begins can cause difficulties during the process. As an example, the MEG is heated in a heat exchanger and remains hot in the flash separator vessel. Under certain conditions, the heating process may convert some of the hydrocarbons to a black solid coke-like material that remains in suspension inside the flash separator. As more of this material forms, it increases the rich MEG's tendency to form a stable foam, reduces the settling of salt crystals, increases the abrasive nature of the rich MEG so that pump seal failure and pipe erosion are more prevalent, and discolors the salt produced in the reclamation process making it unsuitable for marine discharge. In offshore operations, excessive hydrocarbon levels in the rich MEG feed stream can carry through the MEG reclamation process to the lean MEG and cause plugging of the downstream lean MEG injection nozzles if the hydrocarbons form a solid compound under line or injection conditions.
In addition, hydrocarbons carried with the rich MEG feed stream into the MEG reclamation process must either remain in the flash separator or flash to the distillation column. The light hydrocarbons that flash to the distillation column exit with the distilled water, while the heavy hydrocarbons that flash to the distillation column must exit with the lean MEG. In either case, the hydrocarbons may need to be removed to meet water discharge or product purity specifications.
Hydrocarbon removal beyond the final production separator is currently limited. Activated charcoal filters are generally used for such removal and are capable of reducing the hydrocarbon levels to the range of approximately 25 to 50 ppm. However, because activated charcoal filters are not very efficient, they are heavy and require large amounts of space and volume. This additional space and weight is very expensive, particularly for offshore operations. In addition, the charcoal filter material must be replaced whenever it is fully adsorbed with hydrocarbons. The spent material may be disposed of as waste or regenerated onshore. Periodically replacing the filter material and properly handling the spent material further increase the costs associated with activated charcoal filters.
A need exists for removing hydrocarbons from rich MEG and other gas process feed streams in order to improve the efficiency and eliminate problems during the treatment process. A need also exists for removing hydrocarbons from distilled water that is produced as a by-product of the MEG reclamation process. A need also exists for a hydrocarbon removal bed that is smaller and lighter than the charcoal filters that are currently used and that can be regenerated without removing and processing the adsorbent material.